1. Field of the Invention
Embodiments of the present invention generally relate to peroxide cured elastomer systems.
2. Description of the Related Art
Crosslinked elastomer formulations based on ethylene-propylene (EP) and/or ethylene-propylene-diene (EPDM) elastomers are useful in a variety of applications that are exposed to high (>150° C.) and/or low (<−20° C.) temperatures during the product life time. Such applications include automotive parts like under-hood belts and hoses. To improve flexibility and/or workability of such crosslinked polyolefin elastomer formulations, a substantial amount of process oil, which acts as both a processing aid and a plasticizer, is added. To maintain the mechanical integrity (and therefore performance) of the manufactured part, it is desirable to minimize the gradual loss of the process oil over time.
Permanence is especially challenging when the part is exposed to high temperatures over long times and can be a problem when a low molecular weight oil is used. On the other hand, the parts should also stay flexible to very low temperatures. This presents a need for an oil with a low pour point, which improves as molecular weight is lowered. Thus, the choice of process oil is a balance between permanence (favoring high molecular weight) and low-temperature performance (favoring low molecular weight) and defines a practical temperature window of utility for the formulation (i.e., from the onset of brittleness on the low end to the onset of unacceptable oil loss at the high end).
When under-hood temperature requirements are high, a peroxide cure system is increasingly used for EP and EPDM compounds because traditional sulfur-cured compounds have an upper use temperature of about 130-150° C. This presents another problem for mineral oils, since most contain chemical moieties (cyclic or aromatic structures, unsaturation, and heteroatoms) that can interfere with peroxide cure systems and can compromise the cure rate and final cure state (degree of crosslinking). This also presents a challenge for conventional mineral oils in terms of the permanence/pour-point balance.
WO 02/31044 describes mono-olefinic polyalphaolefin (PAO) oils having molecular weights between 400 and 1,000 g/mol, as plasticizers for vulcanized or unvulcanized olefin elastomers such as EP, EPDM, natural, butyl, and polybutadiene rubbers. U.S. Pat. No. 4,645,791 describes vulcanized elastomer compositions of sulfur cured EP and EPDM rubbers; a reinforcing particulate filler; and a synthetic hydrocarbon basestock lubricating oil with a molecular weight of between 250 and 1,500 g/mol. EP 0315363 describes vulcanized elastomer compositions of sulfur cured EP and EPDM rubbers; a reinforcing particulate filler; a PAO oil or blend of PAO oils with a kinematic viscosity at 100° C. of 2 to 200 cSt; and a naphthenic and/or paraffinic mineral oil. U.S. Pat. No. 4,833,195 describes pelletizable polyolefin compositions of an EP, EPDM, polybutadiene, polyisoprene, and polyisobutylene rubbers; and a PAO oligomer of C2 to C6 olefins having a molecular weight of less than 15,000 g/mol; and optionally a thermoplastic polyolefin such as polyethylene or polypropylene. EP 1028145 describes vulcanized ethylene/propylene/ethylidene-norbornene compositions comprising 300 to 700 phr of a softener that has a low pour point (−40° C. or less) and a solubility parameter of 6 to 8 (units are unstated), such as low molecular weight EP copolymers, mineral oils, or liquid polybutene or polyisoprene. WO 02/18487 and WO 03/048252 describe thermoplastic elastomer compositions with phenolic resin cured EP, EPDM, ethylene/alpha-olefin, butyl, and butadiene rubbers; 10 to 90 wt % of a thermoplastic polyolefin; and a PAO oil with a molecular weight of 1,000 g/mol or less. Mineral oils are added to the composition if at least 25 wt % of the total oil is PAO. WO 04/09699 describes compositions of EPDM rubber and a Fischer-Tropsch derived paraffinic process oil with a kinematic viscosity at 100° C. of between 8 and 30 cSt. Additional references include U.S. Pat. No. 6,451,915; U.S. 2005/222861, and JP 07292167.
Based on the above, there is a need for an oil of highly paraffinic structure that minimizes interference with peroxide cure systems, is thermally stable, is of sufficient molecular weight to increase retention of the oil at high temperatures, and has a low pour point to ensure flexibility at low temperatures.